Structural, Optical and Electrical Properties of NiO Thin films prepared

International Journal of Materials Science and Engineering Vol. 1, No. 2 December 2013

Structural, Optical and Electrical Properties of NiO Thin Films Prepared by Low Cost Spray Pyrolysis Technique S. Sriram and A. Thayumanavan PG and Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur, Tamilnadu, India Email: [email protected], [email protected]

Abstract—Nickel Oxide (NiO) thin films are fabricated using fresh and aged precursor solutions using a low cost simplified spray pyrolysis technique. Structural, optical and electrical properties of two different films were studied. The structural studies show that the film prepared from the aged solution has more grain size (60.3nm) than the film prepared by fresh solution (21nm). From the optical studies it is found that, the band gap of film from fresh solution have 3.6eV while the band gap of film from aged solution is about 3.5eV. The refractive index (n) is measured with the help of PUMA software and for the film from fresh solution it remains at 1.95 through visible region. However, for the film from aged solution, it varies from 2 to 1.78 in visible region. The calculated extinction coefficient (k) values of the films show no significant variation in visible and NIR range. The electrical studies confirm that the grown films are ptype. The resistivity measured for two films shows that the resistivity is low (2.271×102Ω cm) for the film prepared by fresh solution and it is high (2.725×102Ω cm) for the other one.

to fabricate the NiO thin films. The aging effects of the precursor solution on the optical and electrical properties of the grown films are studied and discussed. II. EXPERIMENTAL DETAIL A set of precursor solution is prepared using Nickel acetate tetrahydrate (Ni (C2H3O2)2.4H2O) with 0.1 molarity in ethanol mixed deionized water. One of them is allowed to age about 24 hours and the other one (fresh solution) is taken into use to form NiO film (Sample 1). The second solution (aged solution) is taken after 24 hours and then the second NiO film is fabricated (Sample 2). The volume for both solutions is about 50ml. The substrates used were glasses. The substrates are cleaned in distilled water and then dried using air blower. After that they were cleaned again with acetone in order to remove any strains on it. The solutions were then sprayed manically on the glass substrates with air as carrier gas. The temperature was set to 330ºC. The temperature chosen is due to the acetate in the precursor will decompose above 300ºC. Due to continuous spraying, the substrate temperature is suddenly reduced to 310ºC to 320ºC. Therefore, after every 5 spray, the process was stopped about 5 minutes to recover the substrate temperature to 330ºC. X-ray diffraction analyses were obtained using the model X‟pert PRO (PANalytical) X-ray powder diffractometer with Ni filtered CuKα (1.5406Ǻ) radiation for a range of 10º to 90º 2θ angles. The surface morphology, homogeneity and grain size of the deposited films were studied by SEM model JSM 35 CF JEOL. The optical studies were carried out by Perkin Elmer Lambda 35 UV-VIS-NIR double beam spectrophotometer for a wavelength range of 300nm to 1100nm. Hall effect measurement was carried out with Ecopia HMS 5000 Hall effect measurement system. Thicknesses of the films were measured gravimetrically and the measured thicknesses are 631nm and 676nm for Sample 1 and Sample 2 respectively.

Index Terms—NiO thin films, perfume atomizer, optical studies, refractive index, electrical studies

I.

INTRODUCTION

Nickel oxide (NiO) is a promising p type semiconducting oxide material [1], [2] having a wide band gap of 3.6eV to 4eV [3]. It resembles NaCl structure with octahedral Ni (II) and (O2-) sites [4]. Due to its enormous potential applications such as, antiferromagnetic material [5], [6], chemical sensors [7], electrochromic devices [8], catalysts [9], dye sensitized solar cells (DSSCs) [10], it attracts the researchers attention towards it. NiO thin films were fabricated using many methods such as electron beam evaporation [11], reactive sputtering [12], plasma enhanced chemical vapour deposition [13], pulsed laser deposition [14], spray pyrolysis [15], chemical bath deposition [16] etc. Among various methods, spray pyrolysis is one through which the films can be coated for large area. In this present work a low cost and simplified spray pyrolysis technique using perfume atomizer [17]-[19] is employed

III. Manuscript received September 9, 2013; revised December 30, 2013. ©2013 Engineering and Technology Publishing doi: 10.12720/ijmse.1.2.118-121

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RESULTS AND DISCUSSION


A. Structural Characteristics The x-ray diffractograms of the fabricated NiO films are shown in Fig. 1. The peaks are observed at 35.94, 42.67 and 62.18 2θ values for the first film (Sample 1), and for the second film (Sample 2) 36.362, 43.43, and 62.58 2θ values are observed and they are corresponds to (111), (200) and (220) crystal planes respectively which belong to NiO cubic structure (JCPDS card no. 89-7130).

(Sample 1) exhibit clear and smooth grains. However, the film formed with aging solution (Sample 2) shows greater grain size.

Figure 1. X-Ray difractogram of NiO thin films

Figure 3. Transmission spectra of NiO thin films

The average particle size is calculated using Scherrer formula [20]

The optical band gap of the NiO film samples can be calculated by plotting (αhν) 2 versus photon energy (hν) is shown in Fig. 4. The calculated band gap for Sample 1 is 3.6eV where as for the Sample 2 (from aged solution) it is decreased and it is found to be 3.5eV. From the results, it is observed that, for Sample 2 the grain size is increased due to aging effect of precursor solution and hence results the decreasing band gap.

D=0.91λ/β cos θ

B. Optical Properties Fig. 3 shows the transmission spectra of Sample 1 and Sample 2. For both samples the absorption edge is found around 350nm. The observed transmission for Sample 1 is greater than the Sample 2 which results that the aged solution has lesser transparency.

(1)

where λ wavelength of x-ray, β is the FWHM (full width at half maximum intensity) and θ is Bragg’s angle. The calculated average particle size for Sample 1 is about 21nm and for Sample 2 is 60.3nm.

Figure 4. Optical band gap spectra of NiO thin films

The refractive index (n) of the prepared NiO films have been calculated using PUMA [21], a freeware software based on pointwise unconstrained minimization approach, for a range of wavelength of 300nm to 1100nm. The plot of wavelength versus n is shown in Fig. 5. For the Sample 1, the refractive index of the film remains almost equal through the visible region with the

Figure 2. SEM images of NiO thin films

The microstructures of the prepared NiO thin films are shown in Fig. 2. The SEM images show that the films are formed well with sticky nature with substrate. There are no cracks observed on the film surfaces. It is noted that for the film formed without aging of precursor solution ©2013 Engineering and Technology Publishing

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value of 1.95 which is slightly lesser value from the earlier reported values [22]. For the sample 2, the refractive index value drastically decreases upto 400nm and then it remains constant at 1.77.

where Eg is the energy gap of the film. Using the above relation, the refractive index calculated for sample 1 is 1.95 and for sample 2 is 1.99 from the calculated energy gaps of the film samples. They are in good agreement with the calculated value by PUMA. D. Electrical Properties From the Hall effect measurement it is confirmed that the grown films are p-type. The Table I shows the measured values of Hall coefficient, resistivity, conductivity and mobility of the film samples. TABLE I. ELECTRICAL PROPERTIES OF NIO THIN FILMS

Sample

From the calculated absorption coefficient (α) values the extinction coefficient of the films were calculated over visible and NIR wavelengths using the following formula: (2)

Extinction Coefficient(K)

where λ is the wavelength. A plot between wavelength and absorption coefficient is shown in Fig. 6. From the figure it is clear that the extinction coefficient varies in the uv region. However, it is almost constant in the visible and NIR range for both films. 0.25

0.10 0.05 0.00 600

800

1000

Wavelength Figure 6. Extinction coefficient vs wavelength of NiO thin films

From the calculated values of extinction coefficient, it is clear that the grown films are not strong absorbing medium in the UV and NIR wavelength region.

©2013 Engineering and Technology Publishing

14.73

4.409×10-3 2.271×102 3.342×103

P

Sample 2

676

11.715 3.669×10-3 2.725×102 3.193×103

P

REFERENCES [1]

C. Theoretical Validation of Refractive Index The refractive index of the deposited samples of NiO thin films calculated by PUMA has been validated using the relation between energy gap and refractive index formula: n=√12.417/(Eg-0.365) [24]

631

P type Nickel oxide thin films were fabricated using nickel acetate as precursor solution. The physical properties such as structural, optical and electrical are varying depend upon the aging of precursor solution. If the precursor is allowed to age, the grain size may decrease and hence in decrease in the band gap. The refractive index values are calculated for a range of 300nm to 1100nm. For the film prepared without aging precursor, the refractive index is found to be 1.95 while for the film prepared with aged precursor it varies from 2 to 1.78. The values of extinction coefficient (k) of the films show not much variation in the Visible and NIR range. The electrical studies for the prepared films confirm the p-type conductivity and the resistivity increases when the precursor solution is allowed to age.

0.15

400

Sample 1

IV. CONCLUSION

Sample 1 Sample 2

0.20

RH Type (cm3/C)

μ-mobility, σ-conductivity, ρ-resistivity & RH-Hall coefficient. For Sample 1, the measured resistivity is about 2.271×102Ω cm and for Sample 2 it is about 2.725×102Ω cm. The measured Hall coefficient values for Sample 1 and Sample 2 are 3.342×103 and 3.193×103 respectively. From the observed resistivity of Sample 1 and Sample 2 aging effect of precursor solution increases the resistivity and results in decrease in conductivity. The calculated mobility for Sample 1 is 14.73cm2/Vs and 11.715cm2/Vs for Sample 2.

Figure 5. Variation of refractive index (n) with wavelength of NiO thin films

k=αλ/4π [23]

Thickness μ σ (1/Ω cm) ρ (Ω cm) (nm) (cm2/Vs)

[2]

[3]

[4]

(3)

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S. Sriram has obtained his Masters degree in physics from Bharathidasan University, Tamilnadu, India. His current area in research is Thin Films and Condensed Matter Physics. Dr. A. Thayumanvan obtained his Ph.D degree from Indian Institute of Science, Delhi. He has vast teaching and research experience nearly about 35 years. He guided more than 30 research scholars in the area of Solid State Physics and Condensed Matter Physics. He published more than 50 papers in International, National Journals and conferences. His research area includes Lattice dynamics, Thin Films, and Condensed Matter Physics.

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